Abstract: The lifetime of localized surface plasmon plays an important role in manyaspects of plasmonics and its applications. In small metal nanostructures, thedominant mechanism restricting plasmon lifetime is size-dependent Landaudamping. We performed quantum-mechanical calculations of Landau damping for thebright surface plasmon mode in a metal nanoshell. In contrast to theconventional model based on the electron surface scattering, we found that thedamping rate decreases as the nanoshell thickness is reduced. The origin ofthis behavior is traced to the spatial distribution of plasmon local fieldinside the metal shell. We also found that, due to interference of electronscattering amplitudes from nanoshell-s two metal surfaces, the damping rateexhibits pronounced quantum beats with changing shell thickness.